The present invention relates to a color cathode ray tube device in which the screen section is divided into a plurality of subregions which are separately scanned.
In recent years there has been considerable research aimed at meeting demands relating to high definition color cathode ray tubes or associated large-screen high resolution color cathode ray tubes. One requirement for achieving higher resolution in such tubes is that the electron beam spot on the screen be made smaller. There have also been efforts in the past to improve the electron gun electrode structure and to lengthen and increase the size and aperture of actual electron guns, but results achieved so far have been unsatisfactory. The main reason for this is that the electron gun-to-screen distance becomes larger as a cathode ray tube becomes larger. Thus, the electron lens magnification becomes too large. Accordingly, reducing the electron gun to screen distance is an important aspect of achieving high resolution. Methods for wide-angle deflection are not practical for this purpose, since they result in an increased difference in magnification between central and peripheral portions of the screen.
In the past a system has been proposed for high resolution, large-screen displays in which a plurality of small sized color cathode ray tubes are disposed in horizontal and vertical directions. Such a system is described in Japanese Patent Disclosure No. 48-90428, Japanese Patent Disclosure No. 49-21019 and Japanese Utility Model Disclosure No. 53-117130, etc. A system such as this is effective for very large screen displays such as outdoor displays, etc. that are divided into a large number of portions. However, it is obvious that in an intermediate-scale large-screen display with a screen size of about 40 inches, this system results in reproduction of an image that is uncomfortable to view, since the joints between the images of different regions are visible, due to imprecise alignment of the images. In particular, it is a significant drawback if joints are visible when the system is used in drawing display terminals for computer-aided design.
In response to this drawback, a structure in which the screens of a plurality of horizontally disposed, independent CRT's are combined into a unitary screen section has been proposed in Japanese Utility Model Publication No. 39-25641, Japanese Patent Publication No. 42-4928 and Japanese Patent Disclosure No. 50-17167.
A color cathode ray tube employing a multi-neck system with a unitary screen structure such as this makes it possible to produce an image that is easy to view, since it elimiates the joints between adjacent cathode ray tubes which occur with an array of independent color cathode ray tubes, as described above. However, since the separate scanning regions are extremely close to one another, slight differences in brightness, contrast or hue, etc. become apparent.
Although most people find it difficult to assess images in terms of absolute values, they are very sensitive to relative values and are therefore very conscious of relative differences in the hue, etc. of two images that are placed very close together.
Thus, a multi-neck color cathode ray tube with a unitary screen structure has a major drawback in that it is necessary to have perfectly matching brightness, contrast or hue, etc. in each subregion. In practical terms, the complexity of adjustment and the number of adjustments needed, make this extremely difficult to achieve.
When color displays are provided by tubes such as this, in which the screen section is a unitary section and divided scanning is effected, a color cathode ray tube employing a shadow mask system for color selection permits simple, but sure, color selection and is very practical. The tube with a shadow mask system has the advantage that it does not need the extremely high frequency color switching that is required in an indexing type color cathode ray tube.
However, in color cathode ray tubes employing shadow mask systems that are in general use, three electron guns are provided in a single neck for producing three electron beams in correspondence to red, green and blue phosphors coated on a screen. The practicality of a color display apparatus that is provided with a plurality of such tubes to obtain a unitary screen structure is highly doubtful, since inconsistencies in manufacturing result in non-uniform current emission characteristics (drive characteristics) of the three electron guns. Achievement of perfect matching of the brightness, contrast or hue in the different subregions involves a massive number of adjustments.